1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of these functions.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus such as a copier, a printer, a facsimile machine, etc., includes a latent image carrier on which an electrostatic latent image is formed, and a developing unit to develop the electrostatic latent image with developer. The developed image is then transferred onto a sheet of recording medium and fixed thereon.
To develop electrostatic latent images, two-component developer including toner and magnetic carrier is widely used. While such two-component developer is circulated through the developing unit, the toner is consumed in image development, and a toner supplier supplies toner to compensate for the consumption.
In a known toner supply control method, toner consumption is predicted based on image information that is used by an exposure device to form an electrostatic latent image on the image carrier, and the toner is supplied according to the prediction.
In another known toner supply control method, a toner concentration at a predetermined position is detected with a toner concentration sensor provided on a screw that circularly transports the developer through the developing device, and the toner is supplied so as to adjust the detected toner concentration to a target concentration.
However, in these methods, the toner concentration tends to be uneven in a toner circulation direction in the developing unit, which is hereinafter referred to as toner concentration unevenness. This toner concentration unevenness is further described below with reference to FIGS. 1 through 4.
FIG. 1 is an example of a known developing unit in which such two-component developer is circulated by a first screw 180 and a second screw 110 that transport the developer along a developer circulation path in a direction shown by arrow A.
The developing unit further includes a developing roller 120 facing the second screw 110. At a portion where the second screw 110 and the developing roller 120 face each other, the developer is drawn up to a surface of the developing roller 120 and returned to the developer circulation path after passing through a development area. Further, a toner supply port 170 is located in a portion of the developer circulation path where the second screw 110 is located, and a toner concentration sensor detects changes in toner concentration in the developer at a toner concentration detection position B1.
FIGS. 2 and 3 are graphs illustrating relations between toner supply and toner concentration unevenness when the toner is supplied to the two-component developer at one time and in several batches at intervals, respectively. In each of FIGS. 2 and 3, a vertical axis shows toner concentration, a horizontal axis shows time, a thin solid line is a consumption wave, a dashed line is a supply wave, and a heavy solid line shows toner concentration unevenness.
The consumption waves show results of toner concentration detection when no toner is supplied after a given electrostatic latent image is developed with the two-component developer in which toner concentration is uniform. That is, these consumption waves show examples of toner concentration unevenness or changes in the toner concentration caused by image development.
The supply waves show results of toner concentration detection after toner is supplied to the developer in which toner concentration is uniform. It is to be noted that, in FIG. 3, chain double-dashed lines show waves of individual toner supply that is performed intermittently, and the supply wave shown by a dashed line is created by synthesizing these individual toner supply waves.
The toner concentration unevenness shown by a heavy solid line is created by synthesizing the consumption wave and the supply wave, and shows toner concentration unevenness when toner is supplied to the developer after image development.
As shown by the heavy solid lines in FIGS. 2 and 3, toner concentration becomes uneven after the toner is supplied to the developer, either at one time or in several batches at intervals, in the known methods described above. In particular, toner concentration becomes uneven when the toner is supplied regardless of the consumption wave even if the amount of the toner supplied corresponds to toner consumption, because the toner consumption wave depends on size and location of latent images on the image carrier in actual image formation.
More specifically, the toner concentration in the developer becomes uneven after development of electrostatic latent image if these electrostatic latent images are unevenly distributed on the image carrier. Relations between toner concentration unevenness and uneven distribution of latent images are described below with reference to FIG. 4.
In FIG. 4, the transport direction of the developer by the second screw 110 is shown by arrow A1, and a direction of movement of the surface of the image carrier is shown by arrow A2. Three image patterns formed on sheets of recording media are shown in an upper portion of FIG. 4, and electrostatic latent images corresponding to these image patterns are formed on the image carrier and developed with the developer in which toner concentration is uniform. Shown in a lower portion of FIG. 4 are toner concentrations detected by the toner concentration sensor when no toner is supplied after these latent images are developed.
As shown in FIG. 4, the consumption waves that show toner concentration unevenness depend on distribution of the latent images on the image carrier. It is to be noted that the consumption wave of the image pattern on the right in FIG. 4 is broader than that of the image pattern on the left because a distance between the position where the developer returns to the developer circulation path after passing through the development area and the toner concentration detection position B1 shown in FIG. 1 is longer in the case of the right image pattern than in the case of the image pattern on the left. Accordingly, the developer is agitated for a longer time period, and thus toner concentration is equalized better before the toner concentration detection in the case of the right image pattern than in the case of the left image pattern.
To resolve such toner concentration unevenness, a developing unit according to another toner supply control method has been proposed. The developing unit includes a plurality of toner suppliers each supplying toner from a different supply port. In this supply control method, density distribution of image data is analyzed using histogram analysis, and an amount of toner supplied through each toner supply port is independently controlled according to results of the analysis.
However, because such a developing unit requires a plurality of driving sources to drive the toner suppliers independently and simultaneously, its cost is relatively high and the developing unit is relatively large.